The present invention relates to an improved streaming current detector.
It is well known that most waters contain ions and other charged species such as colloids that give rise to a zeta potential on the surfac eof objects that form flow channels for these waters. When such a surface, in the presence of charged species in water, is moved past a pair of electrodes, a current is generated which is called the streaming current. This current is proportional to the net charge density of the water flow.
The measurement of the streaming current has long been recognized as useful in controlling the amount of chemicals needed to treat water for consumption or disposal. In order to make water clean enough for use, treatment chemicals are employed to change the charge density of the water so that contaminates in the water will form aggregates and settle out of the water as a floc. A variety of chemicals are used to clean water which may have a wide range of contaminates. In particular, raw water that is usually negatively charged is processed with coagulant chemicals such as alum to reduce the negative charge. In all cases, it is economically desirable to minimize the use of the chemicals for floc formation.
Once water has been chemically treated, any floc which forms has a tendency to stick to every surface with which it comes into contact, especially to horizontal surfaces. Unfortunately, if any floc is allowed to build up on a surface disposed proximate a probe in which the streaming current is being measured to evaluate the charge density of a flow stream, the sample will give an unreliable reading of the streaming current.
Many attempts have been made at keeping the probe and its electrodes clean during operation. One of the lastest is that of Canzoneri et. al., U.S. Pat. No. 4,446,435. Canzoneri uses an ultrasonic cleaner attached to a probe to clean the area around it during operation. The use of ultrasonic cleaning has been a partial success since it does tend to remove colloidal particles from the surfaces of the piston-electrode chamber where measurements upon a sample are made. However, Canzoneri failed to realize that his method of introducing the sample flow into the bottom of the housing of the probe and discharging this flow from the housing near the top of the probe was working against the natural flow of the floc with the result that the sample tended to be self-contaminating. Canzoneri, aware that his ultrasonic cleaner did not completely solve the electrode contamination problem, introduced a second invention in U.S. Pat. No. 4,449,101. In the latter device, he included a periodic wash in which a cleaning fluid was backflowed into the piston-electrode chamber. Although this backflow also helped to alleviate the contamination problem, its inclusion made the indicator system considerably more complicated.
A device in which the sample stream is introduced at the top of the probe and discharged downwardly from its side was disclosed by one of the earlier pioneers, Gerdes in U.S. Pat. No. 3,368,145. However, Gerdes allowed the sample stream to enter a reservoir about the pistonelectrode chamber within the probe so that some settling tended to occur before the sample flowed into the chamber.
Moore, U.S. Pat. No. 4,297,640, realized that noise is generated in the signal due to a buildup of floc near the top of the upper electrode as occurred in Gerdes' device. In order to minimize these effects, Moore placed a grounding electrode above the two sensing electrodes and between them and the point of floc buildup. This approach, while offering a remedy to a poorly designed sample flow inlet, greatly reduced the current that could have been produced in the absence of a grounding electrode resulting in a weaker signal.